1. Field of the Invention
The present invention relates to techniques for polishing wafers such as semiconductor wafers by use of a chemical mechanical polishing (CMP) apparatus, and more particularly to a pad conditioner for a CMP apparatus capable of performing a high-frequency oscillation along with a rotation to remove particles of a polishing material, supplied in the form of a slurry in a wafer polishing process, glazed in voids formed in the surface of a pad, thereby achieving an improvement in polishing efficiency.
2. Description of the Prior Art
A CMP method, which has recently been developed, is being highlighted as a next-generation semiconductor wafer machining method. In accordance with the CMP method, a slurry is interposed between a wafer to be machined and a polishing pad so as to achieve chemical and mechanical removal of protruded surface portions of the wafer. Such a CMP method will now be described in detail in conjunction with a general CMP apparatus shown in FIG. 1.
As shown in FIG. 1, the CMP apparatus includes a turntable 1 provided with a smooth upper surface having a substantially circular shape. A polishing pad 2 is attached to the upper surface of the turntable 1. A wafer carrier 4, to which a wafer 3 to be machined is mounted, is arranged above the polishing pad 2. In the process of polishing the wafer 3 using the CMP apparatus having the above-mentioned configuration, the wafer carrier 4 carried with the wafer 3 is depressed by pressure P against the upper surface of the polishing pad 2 while a slurry 5 is continuously supplied to the central portion of the upper surface of the polishing pad 2. Under this condition, both the wafer carrier 4 and turntable 1 rotate, thereby polishing the wafer 3.
In order to effectively supply the slurry 5 between the wafer 3 and polishing pad 2, the wafer carrier 4 oscillates horizontally, along with the simple rotation of both the turntable 1 and wafer carrier 4.
The slurry 5, which is supplied between the surface of the wafer 3 to be machined and the upper surface of the polishing pad 2, has a composition consisting of fumed silica grains, having a grain size of tens of millimeters to several hundred millimeters, suspended in an alkali aqueous solution, such as KOH, exhibiting a superior etchability to the wafer 3. As the wafer 3 is depressed, the slurry having the above-mentioned composition is changed into a colloidal silica gel. During relative motions of the turntable 1 and wafer carrier 4, SiO.sub.2 grains of the colloidal silica gel are fused on the surface of the wafer 3 and then peeled off along with protruded surface portions of the wafer 3, thereby chemically and mechanically removing those protruded surface portions of the wafer 3.
The polishing pad 2 comprises a urethane pad consisting of, for example, a flexible non-woven fabric impregnated with foamed urethane. Such a urethane pad has a plurality of fine voids at the surface thereof. The slurry is received in the voids of the pad so that the pad functions to chemically and mechanically polish the wafer.
In the process of polishing the wafer using the CMP apparatus having the above-mentioned configuration, upright sharp points on the surface of the pad may be worn or laid low by the pressure applied from the wafer to the pad and the oscillation of the wafer. The voids of the pad may also be chocked with the mixture of the slurry and the wafer material separated from the wafer due to the wear of the wafer. That is, a glazing phenomenon occurs.
Where the wear of sharp points on the surface of the pad or the glazing phenomenon occurs, it is difficult for the pad to hold the slurry, thereby degrading the wafer polishing efficiency and the uniformity of the polished wafer surface.
In order to solve such a problem, a method has been proposed, in which the pad is ground at its surface using an electro-deposited diamond disc after being used for several wafers or tens of wafers to remove a surface layer laminated on the pad, thereby removing a glazing phenomenon or other undesirable phenomenons occurring at the pad. That is, a new pad surface is periodically formed in accordance with this method. Thus, a continuous wafer machining is carried out.
However, diamond grains separate from the diamond disc during the conditioning of the pad. Such separated diamond grains form scratches on the surface of a wafer being polished. Furthermore, the pad and wafers may be contaminated by metal grains separated from the disc or a bonding material (SUS or Ni) interposed between the disc and diamond. In some cases, the contaminated pad should be replaced by a new one. However, this results in an increase in costs because the pad is expensive.
Furthermore, when the pad is ground by the diamond disc, a large amount of the pad material is removed. That is, the pad is severely worn. As a result, the life of the pad is shortened. In addition, a lot of time is taken for the conditioning of the pad. This results in a degradation in yield.